铷原子热蒸气中强非线性效应产生激光模式图样的实验研究

研究了具有高斯横向分布的连续激光束单次通过铷原子蒸气后,在近共振附近铷原子蒸气中,由强的非线性克尔效应导致激光分裂成细丝的现象,并且这些细丝的衍射图样在远场通过相干叠加,可以形成具有规则结构的斑图模式.实验上研究了输入光功率,铷泡温度和抽运激光频率相对于⁸⁵Rb原子D₂线的不同失谐位置等因素对斑图模式的影响.由于铷原子的超精细能级结构,在铷原子蒸气中同时存在与三阶非线性效应相关的四波混频现象,通过扫描探测光的频率同时观察到具有斯托克斯和反斯托克斯光子的拉曼增益现象. 关键词： 铷原子蒸气 克尔效应 自聚焦 斑图     

单波长泵浦双光子吸收铷蒸汽激光器理论分析

双光子吸收碱金属蒸汽激光器（TPAL）在基础研究和国防工程中有重要的应用前景,近些年来已成为激光领域研究热点之一,但TPAL还缺少相关的理论模型。因此,本文基于碱金属原子的双光子吸收能级跃迁过程构建速率方程,并建立了TPAL理论模型,研究了单波长泵浦双光子吸收铷蒸汽激光器（Rb-TPAL）的工作特性,分析了泵浦光束腰位置、蒸汽池温度以及泵浦功率对Rb-TPAL蓝光输出特性的影响。结果表明,通过优化泵浦光束腰位置和蒸汽池温度,在高功率泵浦情况下,Rb-TPAL可获得高功率蓝光激光输出。     

碘激光的大气衰减特性

碘激光波长处在大气微窗口，水汽分子的吸收是碘激光在大气中传输时衰减的主要原因。利用HITRAN数据库，计算了碘激光波长模式的大气衰减情况。利用高分辨串光谱实验系统，获得了1．315μm附近的谱线参量。在我国，由南向北，由夏到冬，水汽浓度逐渐减少，水汽分子的吸收率递减。碘激光有6条超精细谱线，单一最强线频率的碘激光的大气分子吸收比多谱线的碘激光小，更有利于大气传输。在中纬度夏季，单一最强线垂直向上全程的大气分子吸收率与多谱线碘激光水平传输1km相当，为9．0％。碘激光垂直向上传输比海平面水平传输受大气影响小。     

Measurement of laser beam transmittance through laser produced vapour plume

Laser produced plume, consisting of vapor front and ejected substrate on the workpiece surface, is an intermediate between the incident laser beam and the workpiece on which the beam is directed. It partially blocks, defocuses, absorbs, scatters, and deflects the incident beam and thereby reduces the laser energy reaching the workpiece. However, plume additionally acts as a heat source enhancing the machining. Consequently, laser induced plume plays an important role in laser machining process. The present study investigates the transmittance of a reference beam by the plume generated during the laser-workpiece interaction. To achieve the transmittance measurements, a Nd:YAG laser was used and four different materials were employed in the experiment. To obtain realistic values of the plume transmittance in relation to the laser machining process, the reference beam was sampled from the incident laser beam. The study was extended to include the effect of the laser pulse parameters on the transmission process. It is found that about 10–30% of the transmittance of the reference beam through a vapor plume, produced due to laser ablation, occurs close to the workpiece surface.     

碱蒸气激光半导体泵浦源谱宽压窄

 笑脸效应严重制约大功率半导体激光器输出谱宽压窄。为避免笑脸效应,以中心波长780 nm单宽面源大功率半导体激光器为研究对象,利用利特曼外腔结构,优化外腔参数,在激光器自由运转19 W时,获得11.5 W、外腔效率达61%的窄线宽输出。将窄线宽输出激光通过长25 cm, 80 kPa乙烷、110 ℃的铷池,96.7%的泵浦光被吸收,通过与理论模型对比,推断半导体激光器的输出谱宽为15 GHz(0.03 nm)。     

室温下铯原子气室中6P_(3/2)-8S_(1/2)激发态跃迁的速度选择光谱

以室温下的气室铯原子为介质,采用泵浦探测的方法获得激发态之间的光谱.在泵浦光与探测光相向、同向作用于铯原子得到的激发态超精细光谱有很大的差异.当考虑多普勒效应的影响,其分析结果与实验结果相一致.     

半导体泵浦铷蒸气激光器出光实验

针对中心波长780 nm的商用连续波半导体激光器,使用斩波器将连续激光变为脉冲输出形式,用2400 line/mm的平面全息光栅搭建Littrow外腔将线宽压窄至0.2 nm以下。采用透镜组对线宽压窄后的半导体激光进行光束整形,半导体激光经线宽压窄和光束整形后,经透镜聚焦进长约8 mm的铷蒸气泡进行半导体泵浦碱金属激光实验。首次出光得到17.5 mW的基模线偏振铷激光;在最新的改进实验中,半导体泵浦铷激光输出功率已达到2.8 W。     

Spatial mode cleaning in radically asymmetric strongly focused laser beams

We demonstrate that a femtosecond laser pulse strongly focused in air can produce a highly symmetric damage pattern on glass. This damage pattern contains a series of near-perfect radial rings, with diameters much larger than the predicted focal spot diameter. These rings disappear when the experiment is conducted in vacuum, indicating atmospheric involvement. Surprisingly, the shape and size of the rings seem to be nearly independent of the shape of the generating laser beam, showing dramatic spatial mode cleaning. A “half moon” initial laser mode created by obscuring one side of the round beam produces rings of similar quality to those obtained with the unclipped beam. While spatial mode cleaning has previously been reported in filaments, this is the most dramatic demonstration of the effect that we are aware of. We argue that the effect is due primarily to ionization, in contrast to studies in longer filaments that attribute it to self-focusing.     

激光照射下铝靶表面汽化压力的测量

 本文分析了会聚自由振荡钕玻璃脉冲激光束辐照平面铝靶的汽化特性。在激光功率密度约10 MW/cm²下，由拍摄激光与靶相互作用产生射流发光的时间积分照片测得靶面的汽化压力约3.6 GPa。把时间积分照片与高速分幅照片比较，证明在激光与靶相互作用一段时间后确实存在着马赫盘。     

Polarization patterning by laser-induced phase change in ferroelectric polymer films

Polarization patterns were written on ferroelectric poly(vinylidene-trifluoroethylene) copolymer Langmuir–Blodgett films by heating into the paraelectric phase by laser illumination using two methods. In the first method, patterns were produced by direct writing by a focused continuous-wave laser beam that was moved across the sample. This method affords the greatest flexibility. In the second method, the patterns were generated simultaneously with a pulsed laser using a beam interference method. This method produces highly regular patterns quickly over large areas and affords higher patterning resolution. The patterns produced by both methods could be erased and rewritten, making this technique useful for producing reconfigurable nonvolatile memories, infrared imaging sensors, or acoustic imaging transducers.     

盘型激光焊金属蒸汽特征色调-色饱和度-亮度分析法研究

对于大功率盘型激光焊,金属焊件表面在激光束辐射下强烈汽化并形成等离子体状的金属蒸汽羽状物. 该金属蒸汽羽状物可逆向激光束传输,对激光有明显的屏蔽作用, 降低激光辐射至焊件的能量密度,影响焊接效率和质量. 因此研究金属蒸汽特征变化规律及其与焊接质量之间的关联 ,可实现由金属蒸汽特征实时监测激光焊接状态. 以10 kW大功率盘型连续激光焊接304不锈钢钢板为试验对象, 应用高速摄像机摄取金属蒸汽动态图像,将其转换至色调-色饱和度-亮度空间, 提取金属蒸汽面积、激光束受影响路径长度等相关特征量, 以焊缝熔宽的变化作为衡量焊接状态稳定性的参数. 通过金属蒸汽特征值的均值统计和方差分析,试验证明根据金属蒸汽面积和激光束受影响路径长度等金属蒸汽特征可有效地反映熔宽质量, 从而对焊接状况做出动态评估.     

Spontaneous amplified emission of a Na2 diffuse violet band produced through two-photon excitation of sodium vapor

We report observation of enhanced emission of the diffuse violet band at 436.0 nm when sodium vapor is optically pumped with a pulsed dye laser with output wavelength in the 570–595 nm region. A significant enhanced emission is observed in the forward direction along the propagating laser beam when the laser wavelength is tuned to near the two-photon excited 4d state of the Na atom. At a vapor density higher than 10¹⁵ cm^-3 the diffuse violet emission is predominantly produced through processes associated with the excitation of Na₂. The dependence of the enhanced emission on vapor density and laser power density is presented.     

Raman lidar measurements of tropospheric water vapor over Hefei

L625 Raman lidar has been developed for water vapor measurements over Hefei, China since September 2000. By transmitting laser beam of frequency-tripled Nd:YAG laser, Raman scattering signals of water vapor and nitrogen molecules are simultaneously detected by the cooled photomultipliers with photon counting mode. Water vapor mixing ratios measured by Raman lidar show the good agreements with radiosonde observations, which indicates this Raman lidar is reliable. Many observation cases show that aerosol optical parameters have the good correlation with water vapor distribution in the lower troposphere.     

Time resolved laser-induced plasma dynamics

The structure and evolution of the laser-induced vapor plume and shockwave were measured from femtosecond time resolved shadowgraph images. By changing the wavelength of the probe beam (400 and 800 nm), differences in the opacity of the vapor plume were measured as a function of delay time from the ablation laser pulse. The evolution of the temperature and electron number density during and after the ablation laser pulse were determined and compared for ablation in argon and helium background gases. A laser supported detonation wave (LSD) observed for ablation in argon, blocks the incoming laser energy and generates a high-pressure region above the vapor plume.     

Comments on the problem of anomalous fluorescence of atomic sodium vapor at a wavelength of 589.6 nm

The problem of the laser-induced fluorescence of sodium vapor at a wavelength of 589.6 nm in the case of photoexcitation of a small volume of the medium by a laser beam has been solved numerically. The photon transfer from the zone of excitation to the periphery in the direction perpendicular to the direction of propagation of the laser radiation leads to a significant extension of the volume with a high density of excited atoms. The influence of this factor on the formation of the resonance fluorescence of sodium vapor is discussed.     

注入控制铜蒸气激光方向性时间过程测量

实验研究了注入控制对铜蒸气激光非稳腔输出光束质量的影响，从输出光发散角时间分辨过程的测量结果说明注入加速被注入腔内激光发散角的减小过程。由于被注入腔内放大自发辐射的存在，为得到较好的注入控制效果，注入光的脉宽和进入被注入腔的延时必须恰当。     

激光感生等离子体特性的三维数值模拟

在激光焊接过程中,作用于金属工件表面的高强度激光会引起材料的强烈蒸发,金属蒸汽与入射激光相互怍用,又会引起金属蒸汽部分电离,形成激光感生等离子体。本文采用三维模拟方法,考虑保护气和侧吹气的影响,对激光感生等离子体中的温度与速度分布进行了研究。     

All-optical modulator based on a ferrofluid core metal cladding waveguide chip

We propose a novel optical intensity modulator based on the combination of a symmetrical metal cladding optical waveguide （SMCW） and ferrofluid, where the ferrofluid is sealed in the waveguide to act as a guiding layer. The light matter interaction in the ferrofluid film leads to the formation of a regular nanoparticle pattern, which changes the phase match condition of the ultrahigh order modes in return. When two lasers are incident on the same spot of the waveguide chip, experiments illustrate all-optical modulation of one laser beam by adjusting the intensity of the other laser. A possible theoretical explanation may be due to the optical trapping and Soret effect since the phenomenon is considerable only when the control laser is effectively coupled into the waveguide.     

Underwater optical generation of sound: Oblique incidence

This paper addresses itself to the problem of thermoacoustic generation by an obliquely incident laser beam in a body of water. Far-field directivity pattern of thermoacoustic source (TS) has been studied analytically and numerically. Our study suggests the possibility of getting highly directional sound beams in a simple configuration.